The invention relates to cathode-ray tube oxide cathodes, used as sources of electrons emitted by the thermionic effect, and more particularly to the composition of the metal forming the basis of the cathode.
A conventional oxide cathode consists of a layer of alkaline-earth oxides, such as a mixture of barium oxide (BaO), strontium oxide (SrO) and calcium oxide (CaO) or a mixture of BaO and SrO, which is deposited on a basis metal made of nickel or a nickel alloy and comprising one or more reducing elements, such as magnesium (Mg), aluminium (Al), silicon (Si), chromium (Cr), zirconium (Zr) or any other element capable of reducing oxides. The alkaline-earth oxide mixture may itself be doped with other oxides, such as, for example, Sc2O3 and Y2O3.
A conventional oxide cathode is constructed of a tube made of a Ni alloy (generally Nixe2x80x94Cr) to which a cap made in the basis metal is welded. Deposited on the basis metal is a layer made of a mixture of Ba and Sr carbonates or a mixture of Ba, Sr and Ca carbonates. These carbonates, which are stable in air, are subsequently converted to oxides in the vacuum inside the cathode-ray tube. This oxide layer, heated to a cathode working temperature of approximately 800xc2x0 C., becomes the electron-emitting layer when some of the BaO is converted to barium metal.
The formation of barium metal is maintained by the following mechanisms; the cathode, during operation, is heated to a temperature of approximately 800xc2x0 C., causing the reducing elements to diffuse towards the interface between the nickel and the alkaline-earth oxides. These reducing elements, for example, Mg, Al and Si, constantly react with the barium oxide and reduce it, in order to form barium metal according to the reactions: 
The reducing elements added to the nickel are therefore consumed by the chemical oxidation-reduction reactions with BaO. The lifetime of the cathode is directly connected with the consumption of these elements so that, for each of the addition reducing elements chosen, a minimum content is desirable in order to guarantee a minimum lifetime. Furthermore, it is known that some of the compounds resulting from the Ba reduction reactions described above, such as Ba2SiO4 or BaAl2O4, are so highly stable that they can accumulate at the interface [A. Eisenstein, H. John et al., J. Appl. Phys., T.24, No. 5, p. 631, 1953] between the nickel and the alkaline-earth oxides. These compounds, because of their high resistivity, increase the impedance of the interface, thereby reducing the current density of the cathode. In addition, they degrade the lifetime of the cathode because they permanently accumulate at the interface during operation of the cathode. By accumulating, they limit the diffusion of the reducing elements and thus decrease the reactions between the latter and BaO, which in turn reduces the amount of Ba metal formed, which is necessary for emission [E. S. Rittner, Philips Res. Rep., T.8, p.184, 1953]. Another major drawback is that an excessively high accumulation of these compounds may degrade the adhesion of the alkaline-earth oxides to the nickel.
The invention aims to avoid these drawbacks by properly choosing a composition of the material forming the basis of the cathode, which material consists of a nickel alloy for which the content of reducing elements must be chosen within a defined weight concentration range according to the elements in question. Each reducing element is added to the nickel in a concentration range defined by a lower limit and an upper limit, which range guarantees a long lifetime as well as optimum emission performance and reliability. To achieve this result, the metal alloy according to the invention, intended for the manufacture of cathodes for cathode-ray tubes, mainly comprises nickel, together with magnesium (Mg), the weight concentration CMg of which is between 0.01% and 0.1%. Advantageously, it also includes aluminium, the weight concentration CAl of which satisfies the relationship:
CAlxe2x89xa60.14xc3x97(0.1xe2x88x92CMg),
where:
Cmg is the Mg concentration in the nickel expressed as a percentage by weight;
CAl is the Al concentration in the nickel expressed as a percentage by weight.